In order to check whether reduced postprandial thermogenesis, as found in obese subjects depends on insulin resistance, the study tested whether the thermogenetic response to glucose in a group of obese subjects and a group of normal weight subjects differed from that obtained using an insulin-independent monosaccharide such as fructose. Nine obese subjects and 6 control subjects were included in the study. An oral glucose tolerance and fructose tolerance test (75 g) was performed in all subjects on different days. Energy expenditure was calculated both in basal conditions and during the test (resting metabolic rate: RMR) using indirect calorimetry expressed per kg of lean weight, as assessed using bioimpedance measurement techniques. Blood samples were collected to assay glycemia and insulinemia. Results show that increased RMR induced by glucose was significantly reduced in the group of obese subjects compared to controls. In the same group of obese subjects, RMR was found to be significantly higher following fructose in comparison to the glucose response but did not differ from that in controls. Data confirm the existence of reduced thermogenesis in obese subjects induced by glucose. The fact that this phenomenon was not recorded in the same subjects following the fructose tolerance test, whose metabolism is insulin-independent, supports the hypothesis that reduced glucose-induced thermogenesis in obese subjects may depend on insulin resistance.